1. Field of the Invention
The present invention relates to a semiconductor substrate and a semiconductor device fabricated by using this semiconductor substrate.
2. Description of the Related Art
A semiconductor substrate using a nitride semiconductor is used in a power device or the like which operates with a high frequency and a high output power. In particular, as a device which is suitable to perform amplification in a high-frequency band of microwaves, submillimeter waves, millimeter waves, and others, for example, a high electron mobility transistor (HEMT) or the like is known.
As the semiconductor substrate using the nitride semiconductor, there is known a semiconductor substrate having a buffer layer, a GaN layer, and a barrier layer composed of AlGaN sequentially stacked on a Si substrate.
In a lower layer (a high-resistance layer) in the GaN layer, when electrical resistances in a longitudinal direction and a lateral direction are increased, off characteristics of a transistor can be improved, and longitudinal direction leak is suppressed, thereby realizing a high breakdown voltage. Thus, carbon is doped into the GaN layer, a deep level is formed in a GaN crystal, and n-type conduction is suppressed.
On the other hand, an upper layer in the GaN layer functions as a channel layer and it can be a cause of current collapse (a phenomenon that reproducibility of output current characteristics is degraded) when a level to trap a carrier is formed, and hence concentration of carbon and the like needs to be sufficiently reduced (see Patent Literature 1-3).
Further, Patent Literature 4 discloses that a resistance is increased by adding Fe to a GaN layer, and also discloses that carbon is further added to stabilize an energy level of Fe.
Furthermore, Patent Literature 5 discloses that Fe is added to a GaN layer in order to reduce a parasitic resistance and increase a breakdown voltage.